The invention relates to a hydraulic system, more particularly for motor vehicles having a damping device or at least one pressure restricting valve, wherein at least one closing body which can be moved against the action of an axially active energy accumulator seals a conduit cross-section of the hydraulic system.
Hydraulic systems of this kind are used in particular in motor vehicles as devices for operating brakes, as power steering systems and as a device for operating friction clutches, for example in the force flow between an internal combustion engine and a gearbox and/or an electric machine and a drive train. Damping devices such as illustrated for example in GB 20 32 581, are thereby used to dampen pressure shocks in the pressure medium. Damping devices of this kind have at least one closing body axially biased by an energy accumulator and producing at a predetermined closing pressure the connection between the two connector members. Arrangements of this kind are particularly susceptible to periodically recurring pressure shocks in the pressure medium and react when excited in this way for example with noises such as squeaking noises.
If a pressure restricting valve of this kind is installed in a conduit train with the formation of two individual lines, then there is the further drawback that when filling the hydraulic system with pressure medium the two conduit trains have to be filled individually since the pressure restricting valve sealingly separates the two conduit parts from each other.
The object of the invention is therefore to provide a hydraulic system which dampens vibrations entering into the pressure medium and which has an improved method of operation with regard to reducing the development of noise, is more cost-effective to produce and which is easier to assemble. Furthermore the hydraulic system is to be able to be filled entirely from one filling station.
The foregoing objective is achieved through a hydraulic system which has at least one pressure restricting valve comprising a housing with at least two connections and at least one conduit cross-section which can be closed by a closing body which is axially movable against the action of an energy accumulator, wherein at least a part of the axial travel of the closing body is damped. In this way it is possible to avoid any resonance excitation of the closing body through the pulsating medium, whereas an undamped closing body which is only biased with one energy accumulator can enter into resonance with the pressure medium and thus cause onerous noises.
According to the inventive idea for this purpose the damping device can be formed from an energy accumulator element and at least one friction element. By way of example an energy accumulator which axially biases at least one closing body of a damping device can be provided to act in parallel or serially with a friction device, wherein the case of a parallel method of operation the axial movement of the closing body is combined immediately or after a predetermined axial travel of the closing body with an adjusting friction moment. The friction moment can thereby be adjusted linearly, progressively or degressively by the axial path. Furthermore the friction moment can be from the speed of the axial movement of the closing body.
By way of example energy accumulator elements which axially impinge on the closing body can be coil compression springs, rubber elements and/or compressible fluids which for this purpose can be housed in a practically closed chamber and against which the closing body is moved axially under the volume contraction of the chamber. The energy accumulator is for this purpose advantageously supported fixed on the housing indirectly through a piston or the like or directly, for which suitably formed stops can be provided on the housing, connected in one piece with the housing, or additional component parts can be brought to stop with the housing, so that the closing body can be supported against the action of the energy accumulator. It is evident that any type of energy accumulators can also be installed under pretension or with play in the force flow between the closing body and the housing.
An advantageous friction element according to the inventive idea can be active hydraulicallyxe2x80x94for example hydrodynamically and/or hydrostaticallyxe2x80x94and/or mechanically. Thus for example a friction element can be formed on the basis of a shearing action of a viscous fluid, whereby the viscous fluid can be introduced between a component part of the closing body and a component part fixed on the housing, and in the event of relative movement between the two parts can act as a fluid friction element.
Furthermore when using an internally hollow closing body in conjunction with an axially acting spring element, for example a coil compression spring which engages axially in the closing body and is supported axially on the base of the closing body on one side and on the housing of the pressure restricting valve on the other, an advantageous behaviour can be observed, for example a noise-dampening effect, without having to provide a separate friction device. Tests have shown that the cause for this is an internal friction of the arrangement and/or the position of the point of engagement of the spring element in the closing body so that the connecting point which has been placed axially forwards has positive effects on the tilting behaviour of the closing body. Through this hollow design with an adequate axial guidance of the closing body the connecting point of the spring element can be moved towards the conduit cross-section or sealing edge of the pressure restricting valve. Advantageously the connecting point can be moved into the front half, preferably into the front third of the axial extension of the closing body. Furthermore mass concentrations axially between the connecting point and the front part of the closing body facing the sealing edge can have advantageous effects, for example a shock absorber effect for vibrations in the pressure medium. It is evident that the connection of the spring element inside the closing body can also be advantageous for designs having a friction element.
According to an inventive idea it can be advantageous to arrange at least two pressure restricting valves in a housing wherein these can advantageously be arranged to act opposite one another. The housing can be a separate housing introduced in a conduit, and it is furthermore possible to arrange one or more pressure restricting valves in a functional component part belonging to the hydraulic system, for example a master cylinder and/or a slave cylinder, whereby particularly favourable arrangements can be formed with regard to the number of component parts.
The arrangement of two or more pressure restricting valves in a housing produces in a particularly advantageous way so-called xe2x80x9cvibration filtersxe2x80x9d of high quality since a bi-directional damping of pressure fluctuations in the pressure medium can be filtered effectively and bi-directional. These damping devices can be particularly advantageous if designed and fitted with damped pressure restricting valves according to the inventive idea.
One advantageous development of a hydraulic system of the said type provides at least one closing body which consists of at least two relatively displaceable parts. These parts can engage radially into each otherxe2x80x94and can be formed hollow on the inside so that during displacement of the two parts relative to each other a changing volume can be formed in a chamber enclosed by the two parts. The two parts, for example the closing body itself and a component part, such as for example a piston, radially adjoining the inside walls of the closing body with sealing action, and which can likewise be hollow, can contain a compressible fluid, for example a gas such as air or a foam body, for example formed from compressible hard foam which is reversibly deformable. In this way an energy accumulator element can be contained in the two-part closing body. The friction element according to the inventive idea can be formed mechanical, whereby the walls between the closing body and the piston displaceable axially relative thereto, mechanically form a friction device during a relative movement of the two parts. At least one of the contact faces of the closing body or of the piston can hereby be suitably pretreated or coated in order to set the required friction value of the two parts relative to each other. It can also be advantageous to provide in the fluid or foam body a corresponding inner friction, for example through added friction materials. It is evident that the piston can be supported on a part fixed on the housing against the action of the energy accumulator which is provided between the piston and closing body. Furthermore the second component part or piston or ram can be fixedly connected to the housing and can engage axially into the opening provided on the side opposite the sealing face of the closing body.
Furthermore it can be advantageous to provide instead of the fluid or foam body provided in the chamber, a coil compression spring which is tensioned between the closing body and the component part fixed on the housing such as the ram or piston. A mechanical friction device can also be provided here between the walls of the closing body and the piston, but it is particularly advantageous to fill the ensuing chamber with pressure medium and to provide a connection for the pressure medium between the chamber in the closing body closed by the piston and a chamber which is connected directly to or formed by the conduits of the hydraulic systemxe2x80x94for example a chamber in the region of a valve seat which is closed by the closing bodyxe2x80x94. This connection can then be designed so that a friction element is formed through the shearing action of the pressure medium during displacement of the part fixed on the housing, such as the piston, against the closing body. The connection can thereby be formed from at least one and advantageously from several longitudinal grooves spread out over the interior circumference of the closing body and shaped, for example imprinted, in the closing body and/or in the piston. Through the length, cross-section, shape and/or number of longitudinal grooves it is possible to adjust the friction moment required. Furthermore it can be advantageous to select the coefficients of heat expansion of the two materialsxe2x80x94piston, closing body materialxe2x80x94and to match them with each other so that the temperature-dependence of the viscosity of the pressure medium is at least partially compensated.
Through this arrangement according to the invention during axial displacement of the closing body as the valve opens pressure medium is forced out of the closing body chamber and causes a friction moment as a result of the shearing action in the longitudinal grooves. The same process can arise in the reverse flow direction of the pressure medium during relaxation of the pressure of the medium and therefore during closing of the pressure restricting valve through the closing body.
It can furthermore be advantageous to provide in addition or as an alternative in the closing body an opening having a predetermined diameter through which the pressure medium can escape during displacement of the closing body opposite the piston. This opening, for example a bore, can be provided in the front region of the closing body, for example on a conical or frusto-conical shoulder in the region of the valve seat. Through such arrangements it is possible in particular to produce fluid friction moments which are substantially independent of the temperature-dependence of the viscosity of the pressure medium. A further advantage can be for example a simple production with fewer critical structural tolerances.
A conical or frusto-conical design of the front region of the closing body is particularly advantageous for optimising the control cross-sections since the surface ratio of the surface which is biased by the pressure medium is smaller in the case of a closed valve than when the valve is opened. It has proved particularly advantageous to design the angle of conicity of the conical or frusto-conical end of the closing body in the region of between 20xc2x0 and 90xc2x0, preferably between 20xc2x0 and 25xc2x0. Furthermore it has proved advantageous to arrange the two connections of for example the inlet pipe and outlet pipe at an angle to each other. A angle of 135xc2x0 to 45xc2x0, preferably 90xc2x0, is thereby advantageous between the two connections.
Furthermore according to the inventive idea it is particularly advantageous to design a hydraulic system having at least one pressure restricting valve separating the two conduits from each other and which separates two conduit trains from each other, in order to simplify the filling process with pressure medium prior to start-up, so that the hydraulic system can be filled completely from one filling opening, for example by applying vacuum. For this purpose according to the invention a by-pass is provided between the two conduit trains, namely in the region which under pressure-less conditions is by-passed through the at least one pressure restricting valve with closing body which acts with sealing action on the valve seat. This by-pass is designed so that it only opens at the moment of the filling process and automatically closes when the filling process has been completed.
For this purpose a seal can be provided parallel to the at least one pressure restricting valve which only undertakes its function after contact with the pressure medium. For the seal materials can be provided here which swell up on contact with the pressure medium and thus close a gap serving as the by-pass by swelling on contact with the pressure medium. The seal can thereby be formed so that it leaves a gap free radially on the inside and/or radially on the outside which is closed after a swelling process. Particularly advantageous here is the use of NBR (nitrile butadiene rubber) wherein the pressure medium can advantageously be brake fluid or the like.
Particularly advantageous developments of the hydraulic system according to the invention can be release devices for hydraulically operated friction clutches, brake devices, power steering systems and/or the like. Particularly in the case of clutch release devices it can be advantageous to match the hydraulic system to a damping of frequencies in the range below one kilohertz, preferably between 80 and 60 Hz and more particularly between 80 and 100 Hz, since in this range swaying and/or axial vibrations of the internal combustion engine can be transferred through the crankshaft to the release system.
The novel features that are considered as characteristic of the invention are set forth in particular in the appended claims. The improved apparatus itself, however, both as to its construction and its mode of operation, together with additional features and advantages thereof, will be best understood upon perusal of the following detailed description of certain presently preferred specific embodiments with reference to the accompanying drawing.